Composite superconductor having a core of superconductivity metal with a nonsuperconductive coat



United States Patent US. Cl. 29-1835 1 Claim ABSTRACT OF THE DISCLOSUREA composite superconductor wire comprising a multiplicity of filamentaryconductors of a superconducting material mounted in anon-superconducting carrier is capable of handling a very high currentdensity in a magnetic field in excess of the critical field of thesuperconductive material. The composite wire is produced from a billetof carrier material containing inclusions of the superconductor materialwhich is drawn down so that filamentary conductors of the appropriatesize are produced. The superconductive enclosed element, taken from thegroup of vanadium, lead, niobium and an alloy niobium and zirconium, ismade so small that it remains superconductive even in a field thatexceeds the critical field strength for the material.

This application is a continuation of application Ser. No. 306,346,filed Sept. 3, 1963, now abandoned.

This invention relates to superconducting wire and methods of makingsuch wire.

The theory of superconductivity predicts that fine filaments ofsuperconducting material (less than about 0.1 micron in diameter) shouldbe capable of carrying a high current density (up to 2.10 amps persquare cm.) in magnetic field strengths appreciably in excess of thethermodynamic critical field. However, the difficulties of drawing andhandling such filaments has hitherto prevented the practical realisationof the theory.

According to the present invention a superconducting wire comprises acarrier and at least one inclusion, each inclusion being a continuousfilament of superconducting material of small enough diameter to exhibitsuperconductivity in a magnetic field of a strength in excess of thethermodynamic critical field and the carrier being of a material and ofa diameter capable of being drawn by wire drawing techniques.

The inclusions may be elemental in which case they are preferably ofniobium, although other superconducting materials such as vanadium orlead may be used. Alternatively the inclusions may be alloyed, forexample they may be an alloy of niobium and zirconium. The carriermaterial should be compatible with the filament material, and if thewire is to be annealed at any stage there must not be significantdiffusion of the inclusion into the carrier or vice versa.

The carrier material is not necessarily electrically conducting, but itis preferably non-magnetic and it must not be superconducting at thetemperature the wire is to be used. It is preferable for the carrier tobe an inhibitor of superconductivity and a preferred material ismolybdenum.

Patented Apr. 8, 1969 Two methods of making superconducting wires inaccordance with the present invention will now be described by way ofexample.

In the first method a cylindrical billet of molybdenum about 4 inches indiameter is formed with say equally spaced axial holes which are pluggedwith rods of niobium. The diameter of each rod is 0.04 inch, and theymay simply be inserted in drilled holes in the billet. The compositebillet is then reduced in diameter by swaging and/ or drawing down toabout 0.0005 inch, so that the inclusions are then filaments having adiameter of about 0.000005 inch.

The process may be continued by cutting the wire into lengths, layingthe lengths parallel to make a bundle, which may then be sheathed byextruding molybdenum over it, and swaging and/ or drawing the bundle toreduce the diameter further. This step may be repeated a number of timesuntil, say, the resulting wire is 0.02 inch in diameter and has 10inclusions.

In the second method a bundle is first made up from a large number ofniobium wires which have been electroplated with molybdenum. A sheath ofmolybdenum is extruded over the bundle, and the whole then reduced indiameter as described above.

Although only niobium and molybdenum have been referred to in theparticular examples, other materials may be satisfactorily used subjectto the limitations mentioned above. The best results are howeverobtained where the carrier material is an inhibitor ofsuperconductivity, as is impure molybdenum or is an insulator. This isbecause if the carrier material is merely not superconducting, as forexample copper, there is a tendency when a very fine composite wire hasbeen made for the carrier material in the immediate region of aninclusion to show superconducting properties. This has the effect ofmaking the effective diameters of the filaments greater than theiractual diameters, so reducing the maximum magnetic field strength inwhich the composite wire will have superconducting properties.

I claim:

1. A composite superconductive wire comprising a multiplicity offilamentary conductors of a superconductor from the group consisting ofvanadium, lead, niobium and an alloy of niobium and zirconium, each ofsaid filamentary conductors being of a diameter less than 0.000005 inch,a ductile non-superconducting layer of molybdenum surrounding eachfilamentary conductor and a common outer sheath of a ductilenon-superconductive material.

Current Densities, by Kunzler, Review of Modern Physics, vol. 33, No. 4,October 1961,pp. 501-509.

HYLAND BIZOT, Primary Examiner.

US Cl. X.R.

